Tissue ablation may be used to treat a variety of clinical disorders. For example, tissue ablation may be used to treat cardiac arrhythmias by destroying aberrant pathways that would otherwise conduct abnormal electrical signals to the heart muscle. Several ablation techniques have been developed, including cryoablation, microwave ablation, radio frequency (RF) ablation, and high frequency ultrasound ablation. For cardiac applications, such techniques are typically performed by a clinician who introduces a catheter having an ablative tip to the endocardium via the venous vasculature, positions the ablative tip adjacent to what the clinician believes to be an appropriate region of the endocardium based on tactile feedback, mapping electrocardiogram (ECG) signals, anatomy, and/or fluoroscopic imaging, actuates flow of an irrigant to cool the surface of the selected region, and then actuates the ablative tip for a period of time and at a power believed sufficient to destroy tissue in the selected region.
Although commercially available ablative tips may include thermocouples for providing temperature feedback via a digital display, such thermocouples typically do not provide meaningful temperature feedback during irrigated ablation. For example, the thermocouple only measures surface temperature, whereas the heating or cooling of the tissue that results in tissue ablation may occur at some depth below the tissue surface. Moreover, for procedures in which the surface of the tissue is cooled with an irrigant, the thermocouple will measure the temperature of the irrigant, thus further obscuring any useful information about the temperature of the tissue, particularly at depth. As such, the clinician has no useful feedback regarding the temperature of the tissue as it is being ablated or whether the time period of the ablation is sufficient.
Moreover, during an ablation procedure it is important that the clinician position the ablative tip directly against the cardiac surface (e.g., makes good contact) before activating the ablation energy source and attempting to ablate the tissue. If the clinician does not have good tissue contact, ablation energy may heat the blood instead of the tissue, leading to the formation of an edema, e.g., a fluid-filled pocket or blister on the tissue surface. Such an edema may inhibit adequate destruction of aberrant nerve pathways in the tissue. For example, edemas may physically interfere with the clinician's ability to contact a desired region of tissue with the ablative tip, and thus may interfere with destruction of a desired nerve pathway. Additionally, partial lesions or lesions in undesired locations have been found after the clinician completes the procedure and the edema dissipates. Formation of such partial or undesired lesions are thought to be caused by reduced contact between the ablative tip and the tissue, resulting in a tissue temperature insufficient to cause tissue necrosis. Edemas and partially formed lesions also may make it more difficult to create an effective lesion in the future, for example during a touch-up ablation within the same procedure or later on during a secondary procedure.
Accordingly, it may only be revealed after the procedure is completed—for example, if the patient continues to experience cardiac arrhythmias—that the targeted aberrant pathway was not adequately interrupted. In such a circumstance, the clinician may not know whether the procedure failed because the incorrect region of tissue was ablated, because the ablative tip was not actuated for a sufficient period of time to destroy the aberrant pathway, because the ablative tip was not touching or not sufficiently touching the tissue, because the power of the ablative energy was insufficient, or some combination of the above. Upon repeating the ablation procedure so as to again attempt to treat the arrhythmia, the clinician may have as little feedback as during the first procedure, and thus potentially may again fail to destroy the aberrant pathway. Additionally, there may be some risk that the clinician would re-treat a previously ablated region of the endocardium and not only ablate the conduction pathway, but damage adjacent tissues.
In some circumstances, to avoid having to repeat the ablation procedure as such, the clinician may ablate a series of regions of the endocardium along which the aberrant pathway is believed to lie, so as to improve the chance of interrupting conduction along that pathway. However, there is again insufficient feedback to assist the clinician in determining whether any of those ablated regions are sufficiently destroyed.
U.S. Pat. No. 4,190,053 to Sterzer describes a hyperthermia treatment apparatus in which a microwave source is used to deposit energy in living tissue to effect hyperthermia. The apparatus includes a radiometer for measuring temperature at depth within the tissue, and includes a controller that feeds back a control signal from the radiometer, corresponding to the measured temperature, to control the application of energy from the microwave source. The apparatus alternates between delivering microwave energy from the microwave source and measuring the radiant energy with the radiometer to measure the temperature. As a consequence of this time division multiplexing of energy application and temperature measurement, temperature values reported by the radiometer are not simultaneous with energy delivery.
U.S. Pat. No. 7,769,469 to Carr et al. describes an integrated heating and sensing catheter apparatus for treating arrhythmias, tumors and the like, having a diplexer that permits near simultaneous heating and temperature measurement. This patent too describes that temperature measured by the radiometer may be used to control the application of energy, e.g., to maintain a selected heating profile.
Despite the promise of precise temperature measurement sensitivity and control offered by the use of radiometry, there have been few successful commercial medical applications of this technology. One drawback of previously-known systems has been an inability to obtain highly reproducible results due to slight variations in the construction of the microwave antenna used in the radiometer, which can lead to significant differences in measured temperature from one catheter to another. Problems also have arisen with respect to orienting the radiometer antenna on the catheter to adequately capture the radiant energy emitted by the tissue, and with respect to shielding high frequency microwave components in the surgical environment so as to prevent interference between the radiometer components and other devices in the surgical field.
Acceptance of microwave-based hyperthermia treatments and temperature measurement techniques also has been impeded by the capital costs associated with implementing radiometric temperature control schemes. Radiofrequency ablation techniques have developed a substantial following in the medical community, even though such systems can have severe limitations, such as the inability to accurately measure tissue temperature at depth, e.g., where irrigation is employed. However, the widespread acceptance of RF ablation systems, extensive knowledge base of the medical community with such systems, and the significant cost required to changeover to, and train for, newer technologies has dramatically retarded the widespread adoption of radiometry.
In view of the foregoing, it would be desirable to provide apparatus and methods that permit radiometric measurement of temperature at depth in tissue, and permit use of such measurements to control the application of ablation energy in an ablation treatment, e.g., a hyperthermia or hypothermia treatment, particularly in which contact between the ablative tip and the tissue readily may be assessed.
It further would be desirable to provide apparatus and methods that employ microwave radiometer components that can be readily constructed and calibrated to provide a high degree of measurement reproducibility and reliability.
It also would be desirable to provide apparatus and methods that permit radiometric temperature measurement and control techniques to be introduced in a manner that is readily accessible to clinicians trained in the use of previously-known RF ablation catheters, with a minimum of retraining, and that provide readily understandable signals to the clinicians as to whether the ablative tip is in contact with tissue.
It still further would be desirable to provide apparatus and methods that permit radiometric temperature measurement and control techniques to be readily employed with previously-known RF electrosurgical generators, thereby reducing the capital costs needed to implement such new techniques.